1. Field of the Invention
The present invention relates to an antenna apparatus and a communication apparatus which are used in, for example, cellular phones as well as an antenna apparatus designing method.
2. Related Art of the Invention
First, a configuration and operation of a conventional inverted F antenna (see, for example, the specification of Japanese Patent NO. 1685741, Japanese Examined Patent Application Publication No. H2-13842) will be described mainly with reference to FIG. 13, a perspective view of the conventional inverted F antenna and FIG. 14, its plan view.
The entire disclosure of the document “the specification of Japanese Patent NO. 1685741 (Japanese Examined Patent Application Publication No. H2-13842)” is incorporated herein by reference in its entirety.
The conventional inverted F antenna comprises an antenna element 110 having a feeding plate 111 to which electricity is fed from a feeding point 112, a ground plate 120 arranged opposite the antenna element 110, and a short circuit plate 130 that electrically connects the antenna element 110 and the ground plate 120 together.
The ground plate 120 has a generally rectangular shape in which long sides are each 165 mm in length (length), while short sides are each 44 mm in length (width) (see FIG. 14).
The ground plate 120 has a length of 165 mm, a numerical value which substantially equals the overall length of a folding cellular phone as opened and which corresponds to λ/2, i.e. half of a wavelength λ in a 900-MHz band. Further, the distance between the antenna element 110 and the ground plate 120, H=4 mm (see FIG. 13) is an example of a numeral value required as the height of an antenna incorporated in a folding cellular phone the thickness of which tends to be significantly reduced.
Such a conventional inverted F antenna has such impedance characteristics as shown in FIG. 15(a), a Smith chart illustrating the characteristics of impedance of inputs in view of the antenna from the feeding point of the conventional inverted F antenna (d=13 mm) and in FIG. 15(b) illustrating its VSWR (Voltage Standing Wave Ratio).
However, the above described conventional inverted F antenna has no frequency range within which VSWR≦2 (see FIG. 15(b)) and thus fails to accomplish so called 50-Ω matching. Accordingly, this antenna is very unsuitable for practical use.
Of course, by reducing the distance d between the feeding plate 111 and the short circuit plate 130, it is possible to obtain such impedance characteristics as shown in FIG. 16(a). FIG. 16(a) is a Smith chart illustrating the characteristics of impedance of inputs in view of the antenna from the feeding point of the conventional inverted F antenna (d=2 mm) and in FIG. 16(b) illustrating its VSWR (Voltage Standing Wave Ratio). However, if d=2 mm, resonance frequency is 839 MHz and the frequency range within which VSWR≦2 is between 799 and 872 MHz and hence bandwidth is only 73 MHz. Thus, the specific band obtained by dividing the bandwidth by central frequency is only 8.7%. Accordingly, even if such an inverted F antenna is incorporated in PDC (Personal Digital Cellular) communication equipment that uses, for example, an 800-MHz band for communication, a specific band of 17% or more, required for communication based on this method, is not achieved. Consequently, it is difficult for this antenna to fully cover transmissions and receptions.